Approximate time: 120 minutes

Learning Objectives:

Functional analysis

The output of RNA-seq differential expression analysis is a list of significant differentially expressed genes (DEGs). To gain greater biological insight on the differentially expressed genes there are various analyses that can be done:

Generally for any differential expression analysis, it is useful to interpret the resulting gene lists using freely available web- and R-based tools. While tools for functional analysis span a wide variety of techniques, they can loosely be categorized into three main types: over-representation analysis, functional class scoring, and pathway topology [1].

Pathway analysis tools
Pathway analysis tools

The goal of functional analysis is provide biological insight, so it’s necessary to analyze our results in the context of our experimental hypothesis: FMRP and MOV10 associate and regulate the translation of a subset of RNAs. Therefore, based on the authors’ hypothesis, we may expect the enrichment of processes/pathways related to translation, splicing, and the regulation of mRNAs, which we would need to validate experimentally.

Note that all tools described below are great tools to validate experimental results and to make hypotheses. These tools suggest genes/pathways that may be involved with your condition of interest; however, you should NOT use these tools to make conclusions about the pathways involved in your experimental process. You will need to perform experimental validation of any suggested pathways.

Over-representation analysis

There are a plethora of functional enrichment tools that perform some type of “over-representation” analysis by querying databases containing information about gene function and interactions.

These databases typically categorize genes into groups based on shared function, or involvement in a pathway, or presence in a specific cellular location, or other categorizations, e.g. functional pathways, etc. Essentially, known genes are binned into categories that have been consistently named (controlled vocabulary) based on how the gene has been annotated functionally. These categories are independent of any organism, however each organism has distinct categorizations available.

To determine whether any categories are over-represented, you can determine the probability of having the observed proportion of genes associated with a specific category in your gene list based on the proportion of genes associated with the same category in the background set (gene categorizations for the appropriate organism).

The statistical test that will determine whether something is actually over-represented is the Hypergeometric test.

Hypergeometric testing

Using the example of the first functional category above, hypergeometric distribution is a probability distribution that describes the probability of 25 genes (k) being associated with “Functional category 1”, for all genes in our gene list (n=1000), from a population of all of the genes in entire genome (N=13,000) which contains 35 genes (K) associated with “Functional category 1” [4].

The calculation of probability of k successes follows the formula:

hypergeo
hypergeo

This test will result in an adjusted p-value (after multiple test correction) for each category tested.

Gene Ontology project

One of the most widely-used categorizations is the Gene Ontology (GO) established by the Gene Ontology project.

“The Gene Ontology project is a collaborative effort to address the need for consistent descriptions of gene products across databases” [2]. The Gene Ontology Consortium maintains the GO terms, and these GO terms are incorporated into gene annotations in many of the popular repositories for animal, plant, and microbial genomes.

Tools that investigate enrichment of biological functions or interactions often use the Gene Ontology (GO) categorizations, i.e. the GO terms to determine whether any have significantly modified representation in a given list of genes. Therefore, to best use and interpret the results from these functional analysis tools, it is helpful to have a good understanding of the GO terms themselves and their organization.

GO Ontologies

To describe the roles of genes and gene products, GO terms are organized into three independent controlled vocabularies (ontologies) in a species-independent manner:

  • Biological process: refers to the biological role involving the gene or gene product, and could include “transcription”, “signal transduction”, and “apoptosis”. A biological process generally involves a chemical or physical change of the starting material or input.
  • Molecular function: represents the biochemical activity of the gene product, such activities could include “ligand”, “GTPase”, and “transporter”.
  • Cellular component: refers to the location in the cell of the gene product. Cellular components could include “nucleus”, “lysosome”, and “plasma membrane”.

Each GO term has a term name (e.g. DNA repair) and a unique term accession number (GO:0005125), and a single gene product can be associated with many GO terms, since a single gene product “may function in several processes, contain domains that carry out diverse molecular functions, and participate in multiple alternative interactions with other proteins, organelles or locations in the cell” [3].

GO term hierarchy

Some gene products are well-researched, with vast quantities of data available regarding their biological processes and functions. However, other gene products have very little data available about their roles in the cell.

For example, the protein, “p53”, would contain a wealth of information on it’s roles in the cell, whereas another protein might only be known as a “membrane-bound protein” with no other information available.

The GO ontologies were developed to describe and query biological knowledge with differing levels of information available. To do this, GO ontologies are loosely hierarchical, ranging from general, ‘parent’, terms to more specific, ‘child’ terms. The GO ontologies are “loosely” hierarchical since ‘child’ terms can have multiple ‘parent’ terms.

Some genes with less information may only be associated with general ‘parent’ terms or no terms at all, while other genes with a lot of information be associated with many terms.

Nature Reviews Cancer 7, 23-34 (January 2007)
Nature Reviews Cancer 7, 23-34 (January 2007)

Tips for working with GO terms

clusterProfiler

We will be using clusterProfiler to perform over-representation analysis on GO terms associated with our list of significant genes. The tool takes as input a significant gene list and a background gene list and performs statistical enrichment analysis using hypergeometric testing. The basic arguments allow the user to select the appropriate organism and GO ontology (BP, CC, MF) to test.

Running clusterProfiler

To run clusterProfiler GO over-representation analysis, we will change our gene names into Ensembl IDs, since the tool works a bit easier with the Ensembl IDs.

Let’s open up our previous DEanalysis project, and create a functional_analysis.R script.

Then load the following libraries:

library(org.Hs.eg.db)
library(annotables)
library(enrichplot)
library(DOSE)
library(pathview)
library(clusterProfiler)
library(AnnotationHub)
library(ensembldb)
library(tidyverse)

For the different steps in the functional analysis, we require Ensembl and Entrez IDs. To convert the gene symbols to these IDs, we will use the annotables package and merge the IDs with the DE results.

# Explore the grch38 table loaded by the annotables library
names(grch38)
class(grch38)

res_tableOE = readRDS("data/res_tableOE.rds")

## Return the IDs for the gene symbols in the DE results
idx <- grch38$symbol %in% rownames(res_tableOE)

ids <- grch38[idx, ]

## The gene names can map to more than one Ensembl ID (some genes change ID over time), 
## so we need to remove duplicate IDs prior to assessing enriched GO terms
non_duplicates <- which(!duplicated(ids$symbol))
ids <- ids[non_duplicates, ] 

res_tableOE_tb <- res_tableOE %>%
data.frame() %>%
rownames_to_column(var="gene") %>%
as_tibble()

w = which(res_tableOE_tb$gene%in% ids$symbol)

res_ids = res_tableOE_tb[w,]
m = match(res_ids$gene,ids$symbol)

res_ids$ensgene = ids$ensgene[m]

To perform the over-representation analysis, we need a list of background genes and a list of significant genes. For our background dataset we will use all genes tested for differential expression (all genes in our results table). For our significant gene list we will use genes with p-adjusted values less than 0.05 (we could include a fold change threshold too if we have many DE genes).

## background set of ensgenes
allOE_genes <- res_ids$ensgene

sigOE = dplyr::filter(res_ids, padj < 0.05)
sigOE_genes = sigOE$ensgene

Now we can perform the GO enrichment analysis and save the results:

## Run GO enrichment analysis 
ego <- enrichGO(gene = sigOE_genes, 
                universe = allOE_genes,
                keyType = "ENSEMBL",
                OrgDb = org.Hs.eg.db, 
                ont = "BP", 
                pAdjustMethod = "BH", 
                qvalueCutoff = 0.05, 
                readable = TRUE)
                
## Output results from GO analysis to a table
cluster_summary <- data.frame(ego)

write.csv(cluster_summary, "results/clusterProfiler_Mov10oe.csv")

NOTE: The different organisms with annotation databases available to use with clusterProfiler can be found here Also, the keyType argument may be coded as keytype in different versions of clusterProfiler.

cluster_summary
cluster_summary

Visualizing clusterProfiler results

clusterProfiler has a variety of options for viewing the over-represented GO terms. We will explore the dotplot, enrichment plot, and the category netplot.

The dotplot shows the number of genes associated with the first 50 terms (size) and the p-adjusted values for these terms (color). This plot displays the top 50 genes by gene ratio (# genes related to GO term / total number of sig genes), not p-adjusted value.

## Dotplot 
dotplot(ego, showCategory=50)

To save the figure, click on the Export button in the RStudio Plots tab and Save as PDF.... In the pop-up window, change: - Orientation: to Landscape - PDF size to 8 x 14 to give a figure of appropriate size for the text labels

The next plot is the enrichment GO plot, which shows the relationship between the top 50 most significantly enriched GO terms (padj.), by grouping similar terms together. The color represents the p-values relative to the other displayed terms (brighter red is more significant) and the size of the terms represents the number of genes that are significant from our list.

## Enrichmap clusters the 50 most significant (by padj) GO terms to visualize relationships between terms

pwt <- pairwise_termsim(
ego,
method = "JC",
semData = NULL,
showCategory = 50
)

emapplot(pwt, showCategory = 50)

To save the figure, click on the Export button in the RStudio Plots tab and Save as PDF.... In the pop-up window, change the PDF size to 24 x 32 to give a figure of appropriate size for the text labels.

Finally, the category netplot shows the relationships between the genes associated with the top five most significant GO terms and the fold changes of the significant genes associated with these terms (color). The size of the GO terms reflects the pvalues of the terms, with the more significant terms being larger. This plot is particularly useful for hypothesis generation in identifying genes that may be important to several of the most affected processes.

## To color genes by log2 fold changes, we need to extract the log2 fold changes from our results table creating a named vector
OE_foldchanges <- sigOE$log2FoldChange

names(OE_foldchanges) <- sigOE$gene

## Cnetplot details the genes associated with one or more terms - by default gives the top 5 significant terms (by padj)
cnetplot(ego, 
         categorySize="pvalue", 
         showCategory = 5, 
         foldChange=OE_foldchanges, 
         vertex.label.font=6)
         
## If some of the high fold changes are getting drowned out due to a large range, you could set a maximum fold change value
OE_foldchanges <- ifelse(OE_foldchanges > 2, 2, OE_foldchanges)
OE_foldchanges <- ifelse(OE_foldchanges < -2, -2, OE_foldchanges)

cnetplot(ego, 
         categorySize="pvalue", 
         showCategory = 5, 
         foldChange=OE_foldchanges, 
         vertex.label.font=6)

Again, to save the figure, click on the Export button in the RStudio Plots tab and Save as PDF.... Change the PDF size to 24 x 32 to give a figure of appropriate size for the text labels.

If you are interested in significant processes that are not among the top five, you can subset your ego dataset to only display these processes:

## Subsetting the ego results without overwriting original `ego` variable
ego2 <- ego

ego2@result <- ego@result[c(1,3,4,8,9),]

## Plotting terms of interest
cnetplot(ego2, 
         categorySize="pvalue", 
         foldChange=OE_foldchanges, 
         showCategory = 5, 
         vertex.label.font=6)

Over-representation analysis is only a single type of functional analysis method that is available for teasing apart the biological processes important to your condition of interest. Other types of analyses can be equally important or informative, including functional class scoring and pathway topology methods.

Pathway analysis tools
Pathway analysis tools

Functional class scoring tools

Functional class scoring (FCS) tools, such as GSEA, most often use the gene-level statistics or log2 fold changes for all genes from the differential expression results, then look to see whether gene sets for particular biological pathways are enriched among the large positive or negative fold changes.

The hypothesis of FCS methods is that although large changes in individual genes can have significant effects on pathways (and will be detected via ORA methods), weaker but coordinated changes in sets of functionally related genes (i.e., pathways) can also have significant effects. Thus, rather than setting an arbitrary threshold to identify ‘significant genes’, all genes are considered in the analysis. The gene-level statistics from the dataset are aggregated to generate a single pathway-level statistic and statistical significance of each pathway is reported. This type of analysis can be particularly helpful if the differential expression analysis only outputs a small list of significant DE genes.

Gene set enrichment analysis using clusterProfiler and Pathview

Using the log2 fold changes obtained from the differential expression analysis for every gene, gene set enrichment analysis and pathway analysis can be performed using clusterProfiler and Pathview tools.

For gene set or pathway analysis using clusterProfiler, coordinated differential expression over gene sets is tested instead of changes of individual genes. “Gene sets are pre-defined groups of genes, which are functionally related. Commonly used gene sets include those derived from KEGG pathways, Gene Ontology terms, MSigDB, Reactome, or gene groups that share some other functional annotations, etc. Consistent perturbations over such gene sets frequently suggest mechanistic changes” [1].

To perform GSEA analysis of gene sets, clusterProfiler requires the genes to be identified using Entrez IDs for all genes in our results dataset. We also need to remove the NA values and duplicates (due to gene ID conversion) prior to the analysis:

## Remove any NA values
res_entrez <- filter(res_ids, entrez != "NA")

## Remove any Entrez duplicates
res_entrez <- res_entrez[which(duplicated(res_entrez$entrez) == F), ]

Finally, extract and name the fold changes:

## Extract the foldchanges
foldchanges <- res_entrez$log2FoldChange

## Name each fold change with the corresponding Entrez ID
names(foldchanges) <- res_entrez$entrez

Next we need to order the fold changes in decreasing order. To do this we’ll use the sort() function, which takes a vector as input. This is in contrast to Tidyverse’s arrange(), which requires a data frame.

## Sort fold changes in decreasing order
foldchanges <- sort(foldchanges, decreasing = TRUE)

head(foldchanges)

We can explore the enrichment of BP Gene Ontology terms using gene set enrichment analysis:

# GSEA using gene sets associated with BP Gene Ontology terms
gseaGO <- gseGO(geneList = foldchanges, 
              OrgDb = org.Hs.eg.db, 
              ont = 'BP', 
              nPerm = 1000, 
              minGSSize = 20, 
              pvalueCutoff = 0.05,
              verbose = FALSE) 

gseaGO_results <- gseaGO@result

gseaplot(gseaGO, geneSetID = 'GO:0007423')

There are other gene sets available for GSEA analysis in clusterProfiler (Disease Ontology, Reactome pathways, etc.). In addition, it is possible to supply your own gene set GMT file, such as a GMT for MSigDB using special clusterProfiler functions as shown below:


library(GSEABase)

# Load in GMT file of gene sets (we downloaded from the Broad Institute [website](http://software.broadinstitute.org/gsea/msigdb/collections.jsp) for MSigDB)

c2 <- read.gmt("data/c2.all.v2023.1.Hs.entrez.gmt")

msig <- GSEA(foldchanges, TERM2GENE=c2, verbose=FALSE)

msig_df <- data.frame(msig)

Other Tools

GeneMANIA

GeneMANIA is a tool for predicting the function of your genes. Rather than looking for enrichment, the query gene set is evaluated in the context of curated functional association data and results are displayed in the form of a network. Association data include protein and genetic interactions, pathways, co-expression, co-localization and protein domain similarity. Genes are represented as the nodes of the network and edges are formed by known association evidence. The query gene set is highlighted and so you can find other genes that are related based on the toplogy in the network. This tool is more useful for smaller gene sets (< 400 genes), as you can see in the figure below our input results in a bit of a hairball that is hard to interpret.

genemania
genemania

Use the significant gene list generated from the analysis we performed in class as input to GeneMANIA. Using only pathway and coexpression data as evidence, take a look at the network that results. Can you predict anything functionally from this set of genes?

Resources for functional analysis

This lesson has been developed by members of the teaching team at the Harvard Chan Bioinformatics Core (HBC). These are open access materials distributed under the terms of the Creative Commons Attribution license (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.